It's anxiety. I have the same problem's, although they just started about a month ago. What I do is breath in through your nose and count to 5, and breath out through your mouth and count to 5. Your heart racing is caused by your breathing and your thoughts of dying or having a heart attack, but, if you do my little breathing excercise and DO NOT think about dying it'll go away. Also, remember that marijuana does slightly increase your heart rate.